1. Field of the Invention
The present invention relates generally to timing systems and more particularly to timing and synchronization systems based on global positioning system timing signals.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Many modern technologies rely on extremely precise timing and synchronization. One example of such a technology is the modern cellular telephone system. As most people are aware, cellular telephones and other wireless devices communicate with cellular towers or base stations that are connected to the conventional land-based telephone system or the Internet. Individually, each of these towers only provides coverage for a relatively small area or “cell.” However, by working together, a plurality of towers can create a grid or network of coverage that can encompass an entire city, state, or region. This network of towers is transparent to the end user, because the cellular towers are configured to “hand off” calls from one tower to another tower as the user moves from place to place. For example, if person has a conversation on a mobile telephone while driving to work, this single conservation may actually include a multitude (i.e., five, ten, or more) individual transmissions with different cellular towers along the route. Each of these towers communicates with the wireless telephone while the wireless telephone is in range of that tower and then hands off the call to another tower when the telephone moves out of range. Because the towers are precisely synchronized with each other, the hand off is usually completely transparent to the telephone user. In this way, synchronization enables the “on-the-go” conservations that most people now take for granted.
Precise timing and synchronization is also advantageous in modern power generation and transmission. Electrical power is typically transmitted in the form of three-phase power, which has three separate alternating current (“ac”) power signals that overlap with each other but are out of phase. This three-phase power may be generated by a variety of power plants or sources disposed across an electrical grid. If power generated by one of the power plants is out of synchronization with the power generated by another one of the power plants, the out-of-sync power signals can interfere with each other and reduce the available power. For this reason, modern power generation and transmission facilities typically synchronize three-phase power across the power grid.
One of the fundamental challenges in synchronization is the oscillators that underlie the majority of modern timing systems and clocks. Most modern timing systems employ some form of material, such as quartz crystal or rubidium, in the circuitry used to generate a waveform with a predictable frequency. However, even with the most precise and complex oscillators, there are slight variations in the frequency of the oscillation from one oscillator to another. Over time these slight variations can cause even the most precise oscillator-based clocks or timing systems to become out-of-sync with one another.
One solution for this problem is the atomic clock. Atomic clocks are precision clocks that include an oscillator that is regulated by the natural vibration frequencies of an atomic system, such as the resonance frequency of cesium atoms. Because the resonance frequency of cesium atoms is deterministic and constant, once synchronized, two atomic clocks will maintain virtually the same time (to within a nanosecond or less) for an extremely long period of time. Unfortunately, atomic clocks tend to be fairly expensive, and it is thus not practical to build an atomic clock into every application that could benefit from precise timing and synchronization.
Advantageously, the Global Positioning System (“GPS”) provides a mechanism to distribute precise, atomic clock-based timing data worldwide with only a relatively small number of atomic clocks. As most people are aware, GPS is a satellite-based navigation system that has at least 24 satellites orbiting the earth. These satellites were originally intended for military applications, but have some signals that have been subsequently made available for civilian use. Each GPS satellite contains a highly accurate atomic clock that is synchronized with the atomic clocks on each of the other GPS satellites. Each GPS satellite continually transmits a radio wave signal that includes the current time. A GPS receiver on the surface or in the air can receive this signal and, by comparing the time the signal was transmitted with the time that the GPS receiver received the signal, compute a distance from the GPS receiver to the satellite. By determining the distance between the GPS receiver and at least four satellites, the GPS receiver can triangulate its location.
As described above, GPS receivers determine their distance from GPS satellites by measuring the amount of time that it takes for the signal to be transmitted from the satellite to the GPS receiver. However, because radio waves travel at the speed of light, it may take only nanoseconds (10−9 seconds) for the signal to be transmitted from the satellite to the GPS receiver. As such, in order for the receiver to determine the transmission time accurately, the GPS receiver synchronizes itself to the atomic clocks on the GPS satellites with a degree of accuracy in the nanosecond range. This synchronization is maintained by periodically resynchronizing the GPS receiver with the atomic clock on the GPS satellite.
As described above, the GPS satellites encircle the Earth, and each of the satellites broadcasts a clock signal that is accurate to the nanosecond range. As such, in addition to providing a worldwide location system, the GPS system also provides a highly precise and accurate worldwide clock. For this reason, many of the applications discussed above that depend on precise synchronization use the GPS timing signals for synchronization.
As the number of applications that rely on GPS time for synchronization increases, many people have become concerned about the ramifications if the GPS system were to fail or be shut down. These concerns are especially important in the post-9/11 world, because the United States government has explicitly warned that the civilian GPS system could be turned off in the event of a terrorist act. As such, most GPS based timing devices also include a holdover oscillator that operates in parallel to the GPS system. These holdover oscillators, however, are not as accurate as the atomic clocks on the GPS satellites and, thus, are periodically “tuned” so that the frequency of the holdover oscillator matches the frequency of the atomic resonance of the atomic clocks in the GPS satellites.
Depending on its quality, the holdover oscillator may permit a GPS-based timing system to continue to produce an accurate time for several seconds, minutes, or days, in the absence of a GPS timing signal. The holdover oscillators, however, are dependent on electrical power, and, in the event of a loss of power, the holdover oscillator's “tuning” information can be lost. As such, after a restart, the frequency of the holdover oscillator may not match=the frequency of the atomic resonance in the atomic clocks. If the GPS system is operating normally, the holdover oscillator can immediately be retuned with the GPS timing signal after a restart. However, if the GPS timing signal is not available due to a GPS failure or denial of service, it may not be possible to retune the holdover oscillator to the correct frequency. Without a tuned holdover oscillator, it may be difficult or impossible to restart the application or system that relies on precise synchronization.
A system that can facilitate a restart of a GPS-based timing system in the absence of the GPS timing signal would be advantageous.